Radio waves are characterized in one respect by the way they are polarized, where polarization of a wave is defined as the orientation of the polarity or rotation direction of the electric field. Linear polarization may be horizontal, vertical, or at various angles between the two with respect to the earth's axis or surface. Radio waves also may be circularly polarized either right- or left-hand circular, where the electric field vector rotates in that direction at the rate of the signal frequency.
Standard AM broadcast waves typically have been vertically polarized with respect to the earth. FM broadcast as well as VHF and UHF TV signals are normally horizontally polarized in the United States, but in recent years some applications have used circular polarization in these services. Two-way radio mobile communications such as police, taxi, etc., normally employ vertical polarization.
In the microwave portion of the electromagnetic spectrum, for applications where signals are relayed from tower to tower (e.g., transcontinental microwave links), antennas are oriented for either horizontal or vertical polarization. This method provides improved discrimination between circuits. In addition, dual polarization is often employed on a single antenna in order to obtain twice the normal signal processing capacity available from an antenna with only one polarization.
For satellite communications, both horizontal and vertical polarization is often used on the same satellite, again to double the number of transponders available. A good example of the use of dual polarization on a satellite is the RCA SATCOM IIIR operating in the 4000 MHz region with 24 transponders. The twelve odd numbered transponders (1, 3, 5, etc. . . . ) utilize vertical polarization and the twelve even numbered (2, 4, 6, etc. . . . ) use horizontal polarization. This method of polarization change between adjacent transponders acts to produce increased discrimination and reduces interference that might cause deterioration of the signal from the desired transponder.
At a receiving site on the earth, the "earth station", it is necessary to adjust the receiving antenna's polarization to correspond to that of the transponder from which it is desired to receive signals. Therefore, if the earth station antenna is horizontally polarized and aimed at Satcom IIIR, only the even numbered transponders will be received. Conversely, if the antenna is vertically polarized, then the odd ones will be received. Some earth station antennas have "dual polarized" feeds which are capable of receiving both polarizations simultaneously and thus can receive any or all of the 24 transponders with no further adjustment of the antenna (feed).
Unfortunately, the components required to provide the dual polarized capability for an earth station antenna are expensive, and In some applications, such costs cannot be absorbed by the market. Competition will not withstand the added costs of this equipment.
In the personal earth station market, antennas should be capable of receiving television programs from all of the domestic satellites (domsats) and from all of the transponders on each of the satellites. Thus, the antenna must be capable of responding to either horizontal polarization or vertical, as the case may be, and for some satellites, which have their polarization(s) skewed, the antenna must respond to polarization which is displaced somewhat from truly horizontal or vertical polarization.
The personal earth station market is relatively new. Early designs, utilized a motor driven feed arrangement wherein the entire feed mechanism was rotated physically around the axis which extends from the center of the reflector dish to the focal point. The motor driven mechanism was usually a standard TV antenna rotator easily available on the market and usually designed for outdoor applications and therefore weatherproof. The powered rotator is controlled via a cable which is run to the receiver location. The antenna polarization is typically adjusted at the TV set for best picture as the receiving antenna polarization is driven to coincide with that of the satellite and associated transponder polarization desired.
Since the typical feed assembly for the reflector consists of a feed horn, a section of waveguide, and a low noise amplifier (LNA) plus associated cable, the structure becomes unwieldy and bulky, and difficult to assemble and maintain. In addition, unless great care is taken to have a mechanism which runs true with respect to the axis of rotation, any wobble of the feed horn during rotation will cause the antenna beam to depart from true boresight along the focal axis, and the signal from the satellite will not be in the maximum of the receiving antenna pattern. The quality of TV pictures is therefore degraded. In addition, as actual field installations age, such systems are far from trouble-free, and usually require much repair and maintenance over time.
A far superior arrangement results if the feed horn assembly could be mounted permanently in a fixed position, never to be rotated mechanically. This would eliminate the problem of boresight errors in beam aiming as well as the problems associated with maintenance of mechanical rotators over long periods of time.
The distribution of the electric field within circular waveguide 10 when operating in the dominant TE 1,1 mode is shown in FIG. 1. The lines of electric field, although generally curved symmetrically, are all normal to a plane which passes through the horizontal diameter of the waveguide and extends longitudinally through the waveguide. The horizontal plane can be depicted as a "septum" which in fact can be made of a conducting material such as copper or brass and placed in the waveguide without disturbing the proper operation of the guide. Thus septum 12 will not block or attenuate the wave nor will it cause reflections to occur so long as it is a relatively thin conducting sheet. The septum can be of any length and the wave as it travels through the guide will reform after it has passed by the septum into a wave identical to the original wave. This phenomenon occurs because the electric field lines are at all points perpendicular (normal) to septum 20 and in effect do not "see" the conducting sheet. The wave is said to be cross polarized with respect to the septum.
Another configuration which is functionally identical to the septum or continuous conducting sheet comprises spaced diametric conducting pins which are mounted across the diameter of the circular waveguide in the same plane as the previous septum and spaced along the longitudinal axis of the guide in relatively close proximity. Pin spacings of small fractions of a wavelength can be used. The pins perform the same function with respect to wave propagation as described above for the septum.
If the position of each of the pins described above is rotated slightly around the axis of the circular waveguide, the polarization of the electric field associated with each pin's position therein will tend to remain orthogonal to the pin. If the rotation of each pin is small (a few degrees) so as not to introduce large discontinuities into the structure, a gradual rotation of the polarization will begin and will not upset wave propagation in the waveguide. Such pin configuration is well known and described in greater detail in U.S. Pat. No. 3,864,688. Other methods in the prior art for rotating the polarization of high frequency signals is shown in U.S. Pat. Nos. 3,024,463, 3,599,219 and 3,720,947.
Obviously, in order to adjust the fixed pin configuration described in U.S. Pat. No. 3,864,688 to the polarization of the incident wave from any transponder on any satellite, the entire feed assembly again must be rotated. If the pins themselves are rotated as described in U.S. Pat. Nos. 3,287,729 and 3,296,558, the need for rotating the entire feed assembly is obviated.